Use of nonafluorobutanesulfonic acid in a low ph etch solution to increase aluminum foil capacitance

ABSTRACT

Anode foil, preferably aluminum anode foil, is etched using a process of treating the foil in an electrolyte bath composition comprising a perfluoroalkylsulfonate, a sulfate, a halide, and an oxidizing agent. The anode foil is etched in the electrolyte bath composition by passing a direct current charge through the bath. The etched anode foil is suitable for use in an electrolytic capacitor.

PRIORITY

The present application relates to and claims priority from U.S.provisional application Ser. No. 62/429,444, filed Dec. 2, 2016,entitled “Use of Nonafluorobutanesulfonic Acid in a Low pH Etch Solutionto Increase Aluminum Foil Capacitance,” which is hereby expresslyincorporated by reference in its entirety to provide continuity ofdisclosure.

FIELD OF THE INVENTION

The present disclosure relates generally to methods of using a etchsolutions with particular surface active agents to reduce the tendencyfor overetching and surface erosion during etching of high puritycubicity anode foil. The disclosure also relates to electrolyte bathcompositions for such use, to etched foils produced by such methods, andto electrolytic capacitors.

RELATED ART

Compact, high voltage capacitors are utilized as energy storagereservoirs in many applications, including implantable medical devices.These capacitors are required to have a high energy density since it isdesirable to minimize the overall size of the implanted device. This isparticularly true of an implantable cardioverter defibrillator (ICD),also referred to as an implantable defibrillator, since the high voltagecapacitors used to deliver the defibrillation pulse can occupy as muchas one third of the ICD volume.

Implantable cardioverter defibrillators, such as those disclosed in U.S.Pat. No. 5,131,388, incorporated herein by reference, typically use twoelectrolytic capacitors in series to achieve the desired high voltagefor shock delivery. For example, an implantable cardioverterdefibrillator may utilize two 350 to 400 volt electrolytic capacitors inseries to achieve a voltage of 700 to 800 volts.

Electrolytic capacitors are used in ICDs because they have the mostnearly ideal properties in terms of size and ability to withstandrelatively high voltage. Conventionally, an electrolytic capacitorincludes an etched aluminum foil anode, an aluminum foil or filmcathode, and an interposed kraft paper or fabric gauze separatorimpregnated with a solvent-based liquid electrolyte. The electrolyteimpregnated in the separator functions as the cathode in continuity withthe cathode foil, while an oxide layer on the anode foil functions asthe dielectric.

In ICDs, as in other applications where space is a critical designelement, it is desirable to use capacitors with the greatest possiblecapacitance per unit volume. Since the capacitance of an electrolyticcapacitor increases with the surface area of its electrodes, increasingthe surface area of the aluminum anode foil results in increasedcapacitance per unit volume of the electrolytic capacitor. Byelectrolytically etching aluminum foils, enlargement of the foil surfacearea occurs. As a result of this enlarged surface area, electrolyticcapacitors, manufactured with these etched foils, can obtain a givencapacity with a smaller volume than an electrolytic capacitor whichutilizes a foil with an unetched surface.

In a conventional electrolytic etching process, foil surface area isincreased by removing portions of the aluminum foil to create etchtunnels. While electrolytic capacitors having anodes and cathodescomprised of aluminum foil are most common, anode and cathode foils ofother conventional valve metals such as titanium, tantalum, magnesium,niobium, zirconium and zinc are also used. Electrolytic etchingprocesses are illustrated in U.S. Pat. Nos. 4,213,835, 4,420,367,4,474,657, 4,518,471, 4,525,249, 4,427,506, and 5,901,032.

In conventional processes for etching aluminum foil, an electrolyticbath is used that contains a sulfate, a halide, and an oxidizing agent,such as sodium perchlorate, such as the processes disclosed in U.S. Pat.Nos. 8,871,358, 8,038,866, 7,578,924, 6,858,126, and 6,238,810. Aluminumelectrolytic capacitors' energy density is directly related to thesurface area of the anodes generated in the electrochemical etchingprocesses. Typical surface area increases are 40 fold and represent 30to 40 million tunnels/cm². An electrochemical or chemical widening stepis used to increase the tunnel diameter after etching to insure theformation oxide will not close off the tunnels. Closing off of thetunnels during formation will reduce capacitance and electricalporosity.

It would be advantageous to utilize an etch process, particularly for adirect current (DC) etch process, using agents that prevent closing offof the tunnels during formation and increase foil capacitance and anodestrength.

SUMMARY OF THE INVENTION

The present disclosure provides improved methods and compositions forthe etching of anode foils, as well as etched anode foils provided bysuch methods and compositions. An embodiment of the disclosure providesa method for etching an anode foil by treating the foil in an aqueouselectrolyte bath composition comprising a sulfate, a halide, anoxidizing agent, and a perfluoroalkylsulfonate surface active agent; andpassing a charge through the anode foil while the foil is immersed inthe electrolyte bath. The method includes treating the foil in anaqueous electrolyte bath composition that includes a surface activeagent, such as, e.g., potassium perfluoroalkylsulfonate, and the methodresults in increased foil capacitance.

In any of the embodiments of the disclosure, the anode foil can be firstprecleaned prior to treating the foil in an aqueous electrolyte bathcomposition. Precleaning is conducted by immersing the foil in acorrosive composition, such as hydrochloric acid.

Another embodiment of the disclosure is directed to an aqueouselectrolyte bath composition for etching anode foil. The compositionincludes a sulfate, a halide, an oxidizing agent, and aperfluoroalkylsulfonate surface active agent. The composition caninclude a chloride, such as sodium chloride, potassiumperfluoroalkylsulfonate, and an oxidizing agent such as a perchlorate,e.g., sodium perchlorate.

Another embodiment of the disclosure is directed to an etched anodefoil, provided by a method that comprises adding an etch resist onto theanode foil, treating the anode foil in an aqueous electrolyte bathcomposition comprising a sulfate, a halide, an oxidizing agent, and aperfluoroalkylsulfonate surface active agent, and passing a directcharge through the anode foil while the foil is immersed in theelectrolyte bath, such that the anode foil is etched.

Another embodiment of the disclosure is directed to an electrolyticcapacitor comprising a foil etched by the methods described herein. Afurther embodiment of the disclosure is directed to an ICD comprising acapacitor, wherein the capacitor comprises a foil anode etched by themethods described herein.

It has been discovered that a perfluoroalkylsulfonate surface activeagent, which is thermally and electrochemically stable, can be used inetch processes to obtain a high capacitance yield in a stable etchsolution that is easy to maintain. Accordingly, the present disclosureprovides improved methods and compositions for etching anode foil, aswell as anode foils produced using such methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the foil capacitance versus the concentration ofpotassium nonafluorobutanesulfonate in the etch electrolyte composition,after etching of an aluminum anode foil according to the presentdisclosure.

FIG. 2 illustrates an SEM image of etched foil surface etched in anelectrolyte bath provided according to the present disclosure, preparedusing potassium nonafluorobutanesulfonate.

FIG. 3 illustrates the foil capacitance versus the concentration ofpotassium nonafluorobutanesulfonate in the etch electrolyte composition,after etching of an aluminum anode foil according to the presentdisclosure.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure provides compositions and methods for etching ofanode foils, especially aluminum anode foils, to increase surface areaand capacitance. Several factors contribute to increasing the specificcapacitance of aluminum electrolytic capacitor foil. One factor is theamount of increase in tunnel density (i.e., the number of tunnels persquare centimeter). As tunnel density is increased, a correspondingenlargement of the overall surface area will occur. Another factorcontrolling the increase in specific capacitance is the length of theetch tunnel. Longer tunnels or through tunnels result in higher surfacearea. The tunnel density and tunnel length are both determined by thetype of etch process.

Using the electrolyte bath composition of the present disclosure, thefoil can be etched anodically under the influence of a charge in anelectrolyte bath. In particular, the foil can be etched by treating theanode foil in an electrolyte bath composition comprising a sulfate, ahalide, an oxidizing agent, and a perfluoroalkylsulfonate surface activeagent, and passing a charge through the anode foil while the foil isimmersed in the electrolyte bath. Any and all embodiments of theelectrolyte bath composition may be employed in the methods for etchingof anode foils of the present disclosure.

The electrolytic bath composition of the present disclosure containssulfate (SO₄ ²⁻). The sulfate is provided by a sulfate salt or acid.Suitable sulfate salts and acids include sodium sulfate, potassiumsulfate, lithium sulfate, and sulfuric acid, or other soluble sulfatesalts, with sulfuric acid preferred. The amount of sulfate salt or acidprovided in the electrolytic bath composition can range from about 100parts per million (ppm) to about 2000 ppm (e.g. ranging from about 250ppm to about 1000 ppm). In another embodiment, the sulfate salt or acidis provided in an amount of from about 0.8 to about 1.0% by weight beingpreferred.

The electrolyte bath composition also contains a halide. The halide isprovided by a halide salt, acid, or mixture thereof. The type of halidesalt or acid is not particularly limited, so long as the halide ion isprovided to interact with the sulfate. The halide is believed to helpprovide for pit initiation and tunnel propagation of the anode foil.Suitable halide salts and acids include titanium (III) chloride, sodiumchloride, and hydrochloric acid. A preferred halide salt or acid ishydrochloric acid. The amount of the halide salt or acid added rangesfrom about 1% to about 6% by weight of the electrolyte bath composition,more preferably ranging from about 0.5% to about 3% by weight.

The electrolyte bath composition also contains an oxidizing agent thatis used in conjunction with the halide, provided in the bath by additionof, for example iodic acid, iodine pentoxide, iodine trichloride, sodiumperchlorate, sodium peroxide, hydrogen peroxide, sodium pyrosulfate, andmixtures thereof. Preferably, the oxidizing agent is thermally stableand/or chemically stable, e.g. it is not unduly reduced at the cathode,and helps to create high tunnel density and long tunnels for the etchedfoil. A preferred oxidizing agent is perchlorate, provided by sodiumperchlorate. In one embodiment, sodium perchlorate is used inconjunction with a halide, provided by, e.g., hydrochloric acid.

The amount of oxidizing agent ranges from about 2% to about 12% byweight of the electrolyte bath composition, more preferably ranging fromabout 2% to about 6% by weight. Preferably, the weight ratio ofoxidizing agent to halide is at least about 2 to 1, as measured by theweight of the perchlorate salt and the halide salt or acid used tocreate the bath. In one embodiment, the weight ratio of oxidizing agentto halide is about 2 to 1. In another embodiment, the weight ratio ofoxidizing agent to halide is about 5.6 to 1.

As an example, the amount of sodium perchlorate added can range fromabout 2% to about 12% by weight of the electrolyte bath composition,more preferably ranging from about 2% to about 6% by weight. Similarly,the amount of sodium chloride added can range from about 1% to about 6%by weight of the electrolyte bath composition; more preferably rangingfrom about 1% to about 3% by weight. Illustratively, the weight ratio ofsodium perchlorate added to sodium chloride added is about 2 to 1.

The electrolyte bath composition also contains a perfluoroalkylsufonatesurface active agent. It has been discovered that particular surfaceactive agents increase foil capacitance and lower the amount of etchingcoulombs to achieve an equivalent surface area. In addition, lesssurface erosion on the foil improves the anode strength leading tohigher anode punch yields. Typical surface active agents includeperfluoroalkylsufonate, typically provided as acids or as salts thereof.Perfluoroalkylsufonates are well-known in the art and are readilyavailable from commercial sources (e.g., Sigma-Aldrich Co. LLC.; CharkitChemical Corp.; Mitsubishi Materials Electronic Chemical Co., Ltd.; orFisher Scientific). Preferably, the salt of the perfluoroalkylsulfonicacid is an alkali metal salt or an ammonium salt. More preferably, thesalt of the perfluoroalkylsulfonic acid is a sodium, potassium, lithium,or ammonium salt. Even more preferably, the salt of theperfluoroalkylsulfonic acid is a potassium salt. Preferably, the alkylgroup of the perfluoroalkylsulfonic acid is a C₁-C₈ alkyl group. Morepreferably, the alkyl group of the perfluoroalkylsulfonic acid is aC₁-C₆ alkyl group. Even more preferably, the alkyl group of theperfluoroalkylsulfonic acid is a C₁-C₄ alkyl group. Even morepreferably, the perfluoroalkylsulfonic acid is nonafluorobutanesulfonicacid. In one embodiment, the surface active agent is provided as theacid. In another embodiment, the surface active agent is provided as asalt thereof, such as potassium nonafluorobutanesulfonate.

In an embodiment, the perfluoroalkylsulfonate is provided as an aqueoussolution of C₁-C₈ perfluoroalkylsulfonic acid or a salt thereof. Inanother embodiment, the solution comprising C₁-C₈ perfluoroalkylsulfonicacid or a salt thereof and an aqueous solvent further comprises an aminebase. Suitable amine bases include ethanolamine, diethanolamine,triethanolamine, ethylamine, diethylamine, triethyamine, methylamine,dimethylamine, trimethylamine or ammonia. Preferably the amine base isdiethanolamine. In another embodiment, the solution comprising C₁-C₈perfluoroalkylsulfonic acid or a salt thereof, an aqueous solvent, andamine base has a pH of about 4 to about 9, preferably the pH is about 7.

It is desirable to employ an amount of surface active agent thatincreases foil capacitance, lowers the amount of etching coulombs toachieve an equivalent surface area, and reduces surface erosion on thefoil, improving anode strength leading to higher anode punch yields.Suitable amounts of surface active agent include from about 10 ppm toabout 150 ppm, preferably from about 10 ppm to about 150 ppm. Forinstance, the surface active agent is present in the amount of about 20ppm, about 21 ppm, about 22 ppm, about 23 ppm, about 24 ppm, about 25ppm, about 26 ppm, about 27 ppm, about 28 ppm, about 29 ppm, about 30ppm, about 31 ppm, about 32 ppm, about 33 ppm, about 34 ppm, about 35ppm, about 36 ppm, about 37 ppm, about 38 ppm, about 39 ppm, about 40ppm, about 41 ppm, about 42 ppm, about 43 ppm, about 44 ppm, about 45ppm, about 50 ppm, about 51 ppm, about 52 ppm, about 53 ppm, about 75ppm, about 76 ppm, about 78 ppm, about 100 ppm, about 101 ppm, about 102ppm, about 130 ppm, about 132 ppm, about 133 ppm, about 140 ppm, about142 ppm, about 147 ppm, about 150 ppm, about 151 ppm, about 153 ppm, andabout 155 ppm.

For example, foil capacitance is expected to increase with increasingamounts of surface active agent up to about 150 ppm. Above the 150 ppmlevel, foil capacitance is expected to remain constant or decrease.

An illustrative electrolytic bath composition for use in the presentmethod comprises 20 ppm perfluoroalkylsulfonate provided by an aqueoussolution of 10% potassium nonafluorobutanesulfonate and diethanolamineso that the pH is about 7, chloride provided by 0.62% by weighthydrochloric acid, sulfate provided by 0.92% by weight sulfuric acid,and perchlorate provided by 3.5% by weight sodium perchlorate.

In the method of the present disclosure, the foil can be etchedanodically under the influence of an electrical charge in an electrolytebath, preferably by a direct current (DC). The use of a DC charge willbe discussed below.

Using the method of the present disclosure, foil capacitance isincreased compared to etched foil prepared with an electrolyte bathwithout the C₁-C₈ perfluoroalkylsulfonic acid additive. In an embodimentof the present disclosure, the foil capacitance is increased by about0.5%. In another embodiment of the present disclosure, the foilcapacitance is increased by about 7% to about 8%. In another embodiment,the foil capacitance is increased by about 0.5% or by about 7% to about8% wherein the perfluoroalkylsulfonate is a perfluoroalkylsulfonic acidor salt thereof. In another embodiment, the foil capacitance isincreased by about 3% or by about 7% to about 8% wherein theperfluoroalkylsulfonate is a perfluoroalkylsulfonic acid or saltthereof. In a preferred embodiment, the foil capacitance is increased byabout 4% wherein the perfluoroalkylsulfonate is provided by a potassiumsalt.

The electrolyte bath composition is heated to a temperature ranging fromabout 60° C. and 95° C. (e.g. about 75° C. and about 85° C.), with about80° C. to 81° C. preferred. Illustratively, foil capacitance is expectedto increase with increasing temperature, with a peak capacitanceprovided when the electrolyte bath is heated in the range of about 80°C. to about 81° C.

The foil (preferably a high purity, high cubicity etchable strip assupplied by vendors known to those in the art, and also as discussedbelow) is inserted into the electrolyte bath composition of the presentdisclosure and etched at a DC charge density in an amount ranging fromabout 0.1 to about 0.5 A/cm² (e.g., ranging from about 0.1 to about 0.4A/cm², or from about 0.1 to 0.3 A/cm²), with about 0.15 A/cm² preferred.The etching can be carried out with an etching charge ranging from about20 to about 100 coulombs/cm² (e.g. ranging from about 40 to about 80coulombs/cm², or about 60 to about 80 coulombs/cm², or about 60 to about70 coulombs/cm²), with a range of about 60 to about 70 coulombs/cm²preferred. The time for which the foil is etched ranges from about 2minutes to about 11 minutes (e.g., about 2 minutes, 13 seconds to about11 minutes, 6 seconds), with about 6½ to about 7½ minutes preferred(e.g., about 6 minutes, 40 seconds to about 7 minutes, 47 seconds). Asis understood by those skilled in the art, the etch charge and time willdepend upon the specific applications for which the foil is to be used.

In an embodiment of the disclosure, the etch electrolyte bathcomposition is maintained at a solids level in an amount ranging fromabout 5 g/L to about 40 g/L. For example, when aluminum foil is etchedaccording to the methods of the present disclosure, a portion of thesolid aluminum hydroxide generated during etching may be removed fromthe electrolyte bath composition by passing the composition through amedium with a pore size sufficient to filter the solids to an acceptablelevel. For example, the porous medium may have a pore size ranging fromabout 25 microns and about 40 microns.

In another embodiment of the disclosure, the foil is precleaned prior toetching. By “precleaning” it is meant that the foil, preferably aluminumfoil, is activated by partly removing the natural oxide or contaminationand reveals portions of the fresh aluminum surface on which sulfate ionscan promote tunnel initiation. Proper precleaning prior to etchingresults in an increased capacity for the resulting etched foil.

Precleaning of the foil is accomplished by immersing the foil in acorrosive solution, such as HCl, H₂SO₄, H₃PO₄, or other commerciallyavailable solutions such as the Hubbard-Hall Lusterclean solution for atime sufficient to partly expose the fresh aluminum metal on the foil.For example, the foil can be immersed in an aqueous solution containingHCl in an amount ranging from about 0.1% to about 2% by weight (e.g.from about 0.1 to about 1% by weight, or about 0.2% to about 0.5% byweight), preferably about 0.2% by weight, for a time ranging from about20 seconds to about 2 minutes (e.g. from about 20 seconds to about 1minute), preferably about 20 seconds. The foil is preferably immersed inthe corrosive solution at room temperature (e.g., about 20 to about 30°C.). The foil may then be rinsed with water, preferably deionized water,for at least about one minute.

The foil used for etching according to the present method is preferablyetchable aluminum strip of high cubicity. High cubicity in the contextof the present disclosure is where at least 80% of crystalline aluminumstructure is oriented in a normal position (i.e., a (1,0,0) orientation)relative to the surface of the foil. The foil used for etching is alsopreferably of high purity. Such foils are well-known in the art and arereadily available from commercial sources (e.g., TOYOCHEM CO., LTD. orShowa Chemical Industry Co., Ltd.). Illustratively, the thickness of thealuminum foil ranges from about 50 to about 200 microns, preferably fromabout 110 microns to about 114 microns.

After etching, the foil is removed from the etch solution and rinsed indeionized water. The tunnels formed during the initial etch are thenwidened, or enlarged, in a secondary etch solution, typically an aqueousbased nitrate solution, preferably between about 1% to about 20%aluminum nitrate, more preferably between about 10% to about 14%aluminum nitrate, with less than about 1% free nitric acid. The etchtunnels are widened to an appropriate diameter by methods known to thosein the art, such as that disclosed in U.S. Pat. No. 4,518,471 and U.S.Pat. No. 4,525,249, both of which are incorporated herein by reference.In embodiments of the disclosure, the widening step compriseselectrochemical widening wherein the widening charge ranges from about60 to about 90 coulombs/cm², more preferably about 70 to about 80coulombs/cm².

After the etch tunnels have been widened, the foil is again rinsed withdeionized water and dried. Finally, a barrier oxide layer is formed ontothe metal foil by placing the foil into an electrolyte bath and applyinga positive voltage to the metal foil and a negative voltage to theelectrolyte. The barrier oxide layer provides a high resistance tocurrent passing between the electrolyte and the metal foils in thefinished capacitor, also referred to as the leakage current. A highleakage current can result in the poor performance and reliability of anelectrolytic capacitor. In particular, a high leakage current results ingreater amount of charge leaking out of the capacitor once it has beencharged.

The formation process consists of applying a voltage to the foil throughan electrolyte such as boric acid and water or other solutions familiarto those skilled in the art, resulting in the formation of an oxide onthe surface of the anode foil. The preferred electrolyte for formationis a 100-1000 μS/cm, preferably 500 μS/cm, citric acid concentration. Inthe case of an aluminum anode foil, the formation process results in theformation of aluminum oxide (Al₂O₃) on the surface of the anode foil.The thickness of the oxide deposited or “formed” on the anode foil isproportional to the applied voltage, roughly 10 to 15 Angstroms perapplied volt. The formation voltage can be about 250 Volts or higher,preferably about 250 Volts to about 600 Volts, more preferably about 450Volts to about 510 Volts. The etched and formed anode foils can then becut and used in the assembly of a capacitor.

The present disclosure thus also provides etched anode foil etched bymethods and/or compositions according to the present disclosure. Forexample, the etched foil can be an etched aluminum foil provided by amethod comprising passing a direct charge through an anode foil whilethe foil is immersed in an electrolyte bath, such that the anode foil isetched, wherein the electrolyte bath comprises sulfate provided bysulfuric acid, halide provided by hydrochloric acid, and aperfluoroalkylsulfonate provided by a perfluoroalkylsulfonic acid or asalt thereof, wherein the foil capacitance is increased relative toetched foil prepared with an electrolyte bath without the C₁-C₈perfluoroalkylsulfonic acid additive. The etched anode foil may beetched by any and all embodiments of the electrolyte bath composition.Suitably, the sulfuric acid is provided at about 0.92% by weight, thehydrochloric acid is provided at about 0.62% by weight, the ratio ofhalide to perchlorate is about 5.6:1, the perfluoroalkylsulfonate isprovided at about 10 ppm to about 150 ppm, and the foil capacitance isincreased by at least 0.5% relative to etched foil prepared with anelectrolyte bath without the C₁-C₈ perfluoroalkylsulfonic acid additive.Preferably, the etched foil is provided by a method wherein theperfluoroalkylsulfonate is nonafluorobutanesulfonic acid (FBSA) or saltsthereof. More preferably, the etched foil is provided by a methodwherein the perfluoroalkylsulfonate is provided by potassiumperfluoroalkylsulfonate (KFBS).

The present disclosure thus also provides electrolytic capacitorscomprising etched anode foil etched by methods and/or compositionsaccording to the present disclosure. Such capacitors can be made usingany suitable method known in the art. Non-limiting examples of suchmethods are disclosed, e.g., in the following references which areentirely incorporated herein by reference: U.S. Pat. No. 4,696,082 toFonfria et al., U.S. Pat. No. 4,663,824 to Kemnochi, U.S. Pat. No.3,872,579 to Papadopoulos, U.S. Pat. No. 4,541,037 to Ross et al., U.S.Pat. No. 4,266,332 to Markarian et al., U.S. Pat. No. 3,622,843 toVermilyea et al., and U.S. Pat. No. 4,593,343 to Ross. The rated voltageof the electrolytic capacitor is preferably above about 250 Volts, suchas, e.g. between about 250 Volts and 1000 Volts. Preferably, the voltageis about 400 Volts or higher, more preferably about 400 to about 550Volts. Illustrative capacitance is about 1.0 μF/cm² to about 1.4 μF/cm².

The process of the present disclosure results in a very efficient andeconomical etching process that yields capacitance values equal to orsignificantly higher than available foils, without requiring majorchanges in existing production machinery. The present disclosureprovides improved anode strength, leading to higher anode punch yields.Further, the sulfate ion in the chloride containing solution of thepresent disclosure preferentially adsorbs on the aluminum oxide layer onan aluminum surface of the foil and prevents the chloride ion fromattacking the foil and causing the pitting potential to increase. Oncethe pitting starts, and fresh foil surface is exposed to the etchsolution, the sulfate ion can boost the tunnel growth speed and generatelong tunnels and branch tunnels.

While the above description and following examples are directed to anembodiment of the present disclosure where a sulfate is added to an etchelectrolyte solution to increase the capacitance of aluminum anode foil,sulfate ion can be applied to etch electrolytes to increase thecapacitance of other anode foils known to those skilled in the art. Forexample, the process according to the present disclosure can be used toincrease the capacitance of valve metal anode foils such as aluminum,tantalum, titanium, and columbium (niobium).

Electrolytic capacitors manufactured with anode foils etched accordingto the present disclosure may be utilized in ICDs, such as thosedescribed in U.S. Pat. No. 5,522,851 to Fayram. An increase incapacitance per unit volume of the electrolytic capacitor will allow fora reduction in the size of the ICD.

Having now generally described the disclosure, the same will be morereadily understood through reference to the following examples which areprovided by way of illustration, and are not intended to be limiting ofthe present disclosure.

EXAMPLES Example 1

The effect of surface active agent concentration in an etch electrolytebath compositions on resulting foil capacitance was investigated.

Potassium nonafluorobutanesulfonate was mixed into a low pH etchelectrolyte bath compositions containing chloride and strong oxidizers,prepared by providing about 10 ppm to about 40 ppm potassiumnonafluorobutanesulfonate in an aqueous solution of chloride provided byhydrochloric acid present at 0.62% by weight, sulfate provided bysulfuric acid present at 0.92% by weight, and perchlorate provided bysodium perchlorate present at 3.5% by weight.

FIG. 1 shows the percentage of improvement of foil capacitance over thecontrol (wherein the control was prepared without potassiumnonafluorobutanesulfonate) as a function of the concentration ofpotassium nonafluorobutanesulfonate added, at 475 Volts EFV.

FIG. 2 shows an SEM image of etched foil surface etched in anelectrolyte bath according to Example 1.

Example 2

Nonafluorobutanesulfonic acid was mixed into a low pH etch electrolytebath compositions containing chloride and strong oxidizers, prepared byproviding about 20 ppm nonafluorobutanesulfonic acid in an aqueoussolution of chloride provided by hydrochloric acid present at 0.62% byweight, sulfate provided by sulfuric acid present at 0.92% by weight,and perchlorate provided by sodium perchlorate present at 3.5% byweight.

The etch electrolyte bath compositions containing about 20 ppm ofnonafluorobutanesulfonic acid increased the foil capacitance by 7.0%over the control etch electrolyte bath compositions that did not containany nonafluorobutanesulfonic acid.

Example 3

Diethanolamine was added to a solution of nonafluorobutanesulfonic acidto bring the pH of the solution to about 7. This solution was added at20 ppm of nonafluorobutanesulfonic acid to an aqueous solution ofchloride provided by hydrochloric acid present at 0.62% by weight,sulfate provided by sulfuric acid present at 0.92% by weight, andperchlorate provided by sodium perchlorate present at 3.5% by weight.

The etch electrolyte bath compositions containing about 20 ppm ofnonafluorobutanesulfonic acid and diethanolamine increased the foilcapacitance from 297.7 microF per 270 cm² sheet to 320.7 microF per 270cm² sheet for a 7.2% increase foil capacitance over the control etchelectrolyte bath compositions that did not contain anynonafluorobutanesulfonic acid/diethanolamine solution.

Example 4

Potassium nonafluorobutanesulfonate was mixed into a low pH etchelectrolyte bath compositions containing chloride and strong oxidizers,prepared by providing about 0 ppm to about 150 ppm potassiumnonafluorobutanesulfonate in an aqueous solution of chloride provided byhydrochloric acid present at 0.62% by weight, sulfate provided bysulfuric acid present at 0.92% by weight, and perchlorate provided bysodium perchlorate present at 3.5% by weight.

FIG. 3 shows the foil capacitance as a function of the concentration ofpotassium nonafluorobutanesulfonate added, at 475 Volts EFV.

While various embodiments of the present disclosure have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. Thus, the breadth and scope of thepresent disclosure should not be limited by any of the above-describedexemplary embodiments, but should be defined only in accordance with thefollowing claims and their equivalents. Additionally, all referencescited herein, including journal articles or abstracts, published orcorresponding U.S. or foreign patent applications, issued U.S. orforeign patents, or any other references, are each entirely incorporatedby reference herein, including all data, tables, figures, and textpresented in the cited references.

It must be noted that as used in the present disclosure and in theappended claims, the singular forms “a”, “an”, and “the” include pluralreference unless the context clearly dictates otherwise. Illustratively,the term “a sulfate salt or acid” is intended to include one or moresulfate salts or acids, including mixtures thereof (e.g., sodiumsulfate, potassium sulfate, and/or mixtures thereof) and the term “ahalide salt or acid” is intended to include one or more halide salts oracids, including mixtures thereof (e.g. sodium chloride, potassiumchloride, and lithium chloride, and/or mixtures thereof).

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections may set forth one or morebut not all exemplary embodiments of the present disclosure ascontemplated by the inventor(s), and thus, are not intended to limit thepresent disclosure and the appended claims in any way.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the disclosure that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent disclosure. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of the present disclosure should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

What is claimed is:
 1. An aqueous electrolyte bath composition foretching anode foil, comprising: a sulfate; a halide; an oxidizing agent;and a C₁-C₈ perfluoroalkylsulfonate.
 2. The composition of claim 1,wherein said C₁-C₈ perfluoroalkylsulfonate is present in an amountranging from about 10 to about 150 parts per million (ppm).
 3. Thecomposition of claim 2, wherein said C₁-C₈ perfluoroalkylsulfonate ispresent in an amount ranging from about 20 to about 150 ppm.
 4. Thecomposition of claim 1, wherein the C₁-C₈ perfluoroalkylsulfonatecomprises nonafluorobutanesulfonic acid (FBSA) and/or a salt thereof. 5.The composition of claim 1, wherein the C₁-C₈ perfluoroalkylsulfonate isan alkali metal nonafluorobutanesulfonate.
 6. The composition of claim5, wherein the alkali metal is potassium.
 7. The composition of claim 1,further comprising an amine base.
 8. The composition of claim 7, whereinthe amine base is selected from the group consisting of diethanolamine(DEA), triethanolamine (TEA), and mixtures thereof.
 9. The compositionof claim 1 wherein: the C₁-C₈ perfluoroalkylsulfonate is selected fromthe group consisting of nonafluorobutanesulfonic acid (FBSA), potassiumperfluoroalkylsulfonate (KFBS), and mixtures thereof, present in anamount ranging from about 10 to about 150 ppm, the sulfate is sulfuricacid, wherein said electrolyte bath composition comprises about 0.8% byweight to about 1.0% by weight sulfuric acid, the halide is hydrochloricacid added, wherein said electrolyte bath composition comprises about0.5% by weight to about 3% by weight hydrochloric acid, and theoxidizing agent is sodium perchlorate, wherein said electrolyte bathcomposition comprises about 2% by weight to about 6% by weight sodiumperchlorate.
 10. The composition of claim 9, further comprising an aminebase.
 11. The composition of claim 9, wherein said electrolyte bathcomposition comprises: about 0.92% by weight sulfuric acid; about 0.62%by weight hydrochloric acid; and about 3.5% by weight sodiumperchlorate.
 12. The composition of claim 11, wherein said electrolytebath composition comprises about 20 ppm to about 150 ppm FBSA or KFBS.13. The composition of claim 12, further comprising diethanolamine(DEA).
 14. A method of etching an anode foil, comprising: passing adirect current (DC) charge through an anode foil while the foil isimmersed in an aqueous electrolyte bath, wherein said aqueouselectrolyte bath comprises: a sulfate; a halide; an oxidizing agent; anda C₁-C₈ perfluoroalkylsulfonate; such that the anode foil is etched. 15.The method of claim 14, wherein the C₁-C₈ perfluoroalkylsulfonate isprovided as an aqueous solution of C₁-C₈ perfluoroalkylsulfonic acid ora salt thereof.
 16. The method of claim 15, wherein the solutioncomprising C₁-C₈ perfluoroalkylsulfonic acid or salt thereof and anaqueous solvent further comprises an amine base.
 17. The method of claim16, where in the amine base is diethanolamine or triethanolamine. 18.The method of claim 15, wherein C₁-C₈ perfluoroalkylsulfonic acid orsalt thereof is nonafluorobutanesulfonic acid (FBSA) or potassiumnonafluorobutanesulfonate (KFBS).
 19. The method of claim 14, whereinthe foil capacitance is increased by about 0.5 percent to about 7.2percent relative to electrolyte bath without a C₁-C₈perfluoroalkylsulfonic acid or salt thereof.
 20. An etched anode foil,provided by a process comprising: passing a direct current (DC) chargethrough the anode foil while the foil is immersed in an aqueouselectrolyte bath, such that the anode foil is etched; wherein theaqueous electrolyte bath composition comprises: a C₁-C₈perfluoroalkylsulfonate selected from the group consisting ofnonafluorobutanesulfonic acid (FBSA), potassium perfluoroalkylsulfonate(KFBS), and mixtures thereof, present in an amount ranging from about 10to about 150 ppm; sulfuric acid, wherein said electrolyte bathcomposition comprises about 0.8% by weight to about 1.0% by weightsulfuric acid, hydrochloric acid added, wherein said electrolyte bathcomposition comprises about 0.5% by weight to about 3% by weighthydrochloric acid, and sodium perchlorate, wherein said electrolyte bathcomposition comprises about 2% by weight to about 6% by weight sodiumperchlorate.
 21. The etched anode foil of claim 20, wherein theelectrolyte bath composition further comprises an amine base, whereinthe electrolyte bath composition comprises an amount of the amine basesufficient to neutralize the C₁-C₈ perfluoroalkylsulfonate.
 22. Theetched anode foil of claim 20, wherein the electrolyte bath compositionfurther comprises an amine base selected from the group consisting ofdiethanolamine (DEA), triethanolamine (TEA), and mixtures thereof.